Osteoporosis is a disease that results in the weakening of bone and an increase in the risk of fracture. It has been reported that American females over the age of 50 have about a 50% chance of breaking a bone during their lifetime, and a 40% chance of breaking either a hip, vertebra or wrist. Post-menopausal women lose about 1-3% of their bone mass for each of the first 5-7 years after menopause. Osteoporosis is believed to contribute to about 1.5 million fractures a year in the United States, including about 700,000 spinal fractures and about 300,000 hip fractures. According to the Mayo Clinic, about 25% of the people over 50 who fracture a hip die within a year of the incident. The risk of breaking a bone for an osteoporotic individual doubles after the first fracture. The risk of breaking a second vertebra for an osteoporotic individual increases about four-fold after the first spinal fracture.
Human bone comprises hard mineralized tissue and softer collagenous tissue. The combination of these tissues provides bone with both a structural, weight-bearing capability and a shock-absorption capability. As the bone ages, however, the collagenous portion of the bone is slowly mineralized, thereby making the entire bone more brittle. To compensate for this, bone constantly undergoes a process called “remodeling” in which older, more mineralized bone is replaced by new, more collagenous bone.
Bone remodeling is undertaken by two competing processes: bone formation and bone resorption. Bone formation is largely achieved by bone-forming cells called osteoblasts, while bone resorption is largely achieved by bone-eating (bone-resorbing) cells called osteoclasts. In the normal desired situation, the rate of bone formation is essentially equal to the rate of bone resorption, so that bone mass in the body is maintained.
Osteoporosis occurs when the rate of bone resorption exceeds the rate of bone formation. The rate of bone resorption is largely dependent upon the local production of osteoclasts.
Current treatments for osteoporosis have focused upon arresting the activity of the osteoclast cells. In particular, osteoporosis therapy has focused upon administering drugs called “anti-resorptive agents” or ARA's. The most common classes of anti-resorptive drugs include estrogen, selective estrogen receptor modulators (SERMs), biphosphonates, calcitonin, osteoprotegrin (OPG), cathespin K and statins. Current products include FOSAMAX® (alendronate) in the U.S., Biphosphonate DIDRONEL® (etidronate), and ACTONEL® (risedronate).
Despite the promise provided by these anti-resorptives, there still remain serious issues. First, many anti-resorptives act in a manner that wholly eliminates osteoclast activity. Thus, the delicate balance between bone formation and bone-resorption is again upset, and older, highly mineralized tissue remains within the bone. Although this has the effect of increasing bone mineral density (BMD), the bone that remains is fragile and prone to microdamage.
Second, many of the anti-resorptives are administered systemically, through either oral or intravenous means. Accordingly, side effects associated with systemic administration are often seen. For example, the systemic administration of hormone replacement therapy (“HRT”) has been associated with an elevated cancer risk. In response to this concern, some anti-resorptive drugs, such as biphosphonates, have been engineered to be selective for bone tissue. However, in many cases, the amount of such tissue selective drug that actually reaches bone is often less than 100%.
With respect to the spine, one of the manifestations of osteoporosis is the low pullout strength of pedicle screws. Simply, the lower density of the cancellous bone in the vertebral body reduces the amount of purchase available to a pedicle screw implant.
The art has described a number of different methods for enhancing the pull out strength of pedicle screws. These methods include the use of expandable screws (Cook, Spine Journal, 1(2001) 109-114 and Cook, Spine Journal, 4 (2004) 402-8), and of injectable, settable fluids around the pedicle screw (Bai, Spine, 26(24) 2679-83).